In metal sheet production facilities, especially in steelmaking process lines, phenomena, such as slipping or meandering of a steel sheet and fouling or build-up on surfaces of conveyor rolls, occur when a steel sheet is conveyed by high speed rotation of conveyor rolls. Especially, since hearth rolls for continuous annealing furnaces convey a steel sheet in a high temperature state, the build-up tends to occur on surfaces of hearth rolls. The build-up is a phenomenon in which matters, such as iron or manganese oxide, on a surface of a steel sheet attach to surfaces of hearth rolls and grow. As the build-up proceeds, contaminating objects adhering to the surfaces of hearth rolls gradually grow, and form, for example, projections having diameters of about 100 μm. As a result, protruding shapes of the matters attaching to the surfaces of hearth rolls are transferred to the surface of the steel sheet, to generate recess-shaped defects (referred to as “transferred defects” or “picked-up defects”), as a result of which the quality of the steel sheet deteriorates, and, in addition, removal of matters attaching to roll surfaces is necessary at periodic maintenance, which is a factor that decreases the productivity.
In view of these, various proposals have been made with respect to means for suppressing attachment of contaminating objects to hearth roll surfaces, and, in particular, many of them relate to improvement of the material of a thermally sprayed coating on the surfaces of hearth rolls.
For example, Japanese Patent (JP-B) No. 3234209 discloses a method for producing a sliding member, the method enabling formation a sliding surface having superior anti-sticking properties. This method includes: irradiating a thermally sprayed coating provided on a base with a laser beam in a pattern such as in dots or in lines, thereby partly modifying the coating by heating and structural changes in parts of the coating; and causing laser-irradiated areas or laser-non-irradiated areas to be depressed to form oil pools, by, mainly, selective abrasion during finishing processing or sliding.
Further, Japanese Patent Application Laid-Open (JP-A) No. 2013-95974 discloses a method for forming a densified layer in a thermally sprayed coating, the method including irradiating the surface of a thermally sprayed coating with a high energy beam, thereby causing remelting and resolidification of a coating composition in a surface layer of the thermally sprayed coating and thereby densifying the surface layer.
However, even by utilizing the techniques disclosed in JP-B No. 3234209 and JP-A No. 2013-95974, it is difficult to sufficiently suppress the occurrences of the build-up on the surfaces of hearth rolls, and further improvements in build-up resistance have been desired.